1. Field of the Invention
The present invention relates to an optical device, terminal unit, and system for wavelength division multiplexing.
2. Description of the Related Art
In recent years, the processing of massive amounts of information has been needed with the development of an advanced information society, and optical fiber communication fit for a large capacity has been applied to a transmission network for transmitting information. While a transmission rate of information in optical fiber communication has already reached 2.4 Gb/s or 10 Gb/s, a further increase in transmission capacity will be needed in a motion picture captured communication system that is expected to be put to practical use in the future. For example, a transmission capacity exceeding 1 terabits per second (Tb/s) will be needed in a trunk system.
Research and development on wavelength division multiplexing (WDM) is being actively carried out to increase a transmission capacity in optical fiber communication. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The optical carriers are individually modulated to obtain a plurality of optical signals. The optical signals are next wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is next launched into an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into a plurality of individual optical signals by an optical demultiplexer to reproduce transmitted data according to each optical signal. Accordingly, by applying WDM to such a system, a transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
As an optical device usable as each of the optical multiplexer and the optical demultiplexer, an arrayed waveguide grating (AWG) is known in the art. For example, the AWG includes a first slab waveguide to which a plurality of input optical waveguides are connected, a second slab waveguide to which a plurality of output optical waveguides are connected, and a plurality of connecting optical waveguides provided between the first and second slab waveguides and having different optical path lengths. The connecting optical waveguides and the first and second slab waveguides operate as a diffraction grating, so that each input optical waveguide and each output optical waveguide are coupled by a specific wavelength. Accordingly, by connecting the input optical waveguides to a plurality of optical transmitters, respectively, and connecting one of the output optical waveguides to a single optical fiber transmission line, this AWG functions as an optical multiplexer at a transmitting end. Further, by connecting one of the input optical waveguides to a single optical fiber transmission line and connecting a plurality of optical receivers to the output optical waveguides, respectively, this AWG functions as an optical demultiplexer at a receiving end.
Thus, an AWG may be provided by optical waveguides, so that it is suitable for size reduction of an optical multiplexer and an optical demultiplexer. However, an insertion loss by an AWG is generally as large as about 8 dB, and in the case of applying an AWG to each of an optical multiplexer and an optical demultiplexer in a system adopting WDM, a total insertion loss (associated loss) by the optical multiplexer and the optical demultiplexer becomes about 16 dB.
As a conventional more typical optical device usable as each of an optical multiplexer and an optical demultiplexer, there is an optical device configured by sequentially optically coupling a plurality of optical filters by a reflected light path of a substantially parallel beam. Each of the optical filters is configured by using a filter film such as a multilayer dielectric film. However, the beam diameter of the parallel beam or the Gaussian distribution of its optical power determined in relation to a fiber mode is gradually expanded with an increase in optical path length of the reflected light path. As a result, a deviation in insertion loss between wavelength channels tends to be generated.
It is therefore an object of the present invention to provide an optical device suitable as an optical multiplexer and/or an optical demultiplexer reduced in insertion loss and in deviation in insertion loss. It is another object of the present invention to provide a terminal unit and a system each including such an optical device.
The optical device according to the present invention comprises a fiber collimator and a plurality of fiber assemblies sequentially optically coupled to the fiber collimator by a reflected light path of a substantially parallel beam. The fiber collimator comprises an optical fiber and a lens opposed to the optical fiber. Each of the fiber assemblies comprises an optical filter for providing the reflected light path, an optical fiber optically coupled to the optical filter by its transmitted light path, and a lens provided between the optical filter and this optical fiber.
In the optical device according to a first aspect of the present invention, the lens of the i-th (i is an integer satisfying 2xe2x89xa6ixe2x89xa6N where N represents the number of the fiber assemblies, which is an integer greater than 1) fiber assembly has a focal length equal to or greater than that of the lens of the (ixe2x88x921)-th fiber assembly, and the lens of the N-th fiber assembly has a focal length greater than that of the lens of the first fiber assembly. With this configuration, a deviation in insertion loss due to beam divergence in the reflected light path can be eliminated, thus achieving one of the objects of the present invention.
In the optical device according to a second aspect of the present invention, each optical filter has a reflecting surface for providing the reflected light path, and at least one of the reflecting surfaces of the fiber assembly is formed as a concave surface. With this configuration, a deviation in insertion loss due to beam divergence in the reflected light path can be eliminated, thus achieving one of the objects of the present invention.
In accordance with a third aspect of the present invention, there is provided a terminal unit comprising a plurality of optical transmitters for outputting a plurality of optical signals having different wavelengths, respectively, and an optical multiplexer optically connected to the optical transmitters for outputting WDM signal light obtained by wavelength division multiplexing the optical signals. The optical multiplexer is provided by the optical device mentioned above. Preferably, the optical multiplexer is provided by the optical device according to the first or second aspect of the present invention. The fiber assemblies are optically connected to the optical transmitters, respectively, and the WDM signal light is output from the fiber collimator.
In accordance with a fourth aspect of the present invention, there is provided a terminal unit comprising an optical demultiplexer for receiving WDM signal light obtained by wavelength division multiplexing and for separating the WDM signal light into a plurality of optical signals having different wavelengths, and a plurality of optical receivers optically connected to the optical demultiplexer for receiving the optical signals, respectively. The optical demultiplexer is provided by the optical device mentioned above. Preferably, the optical demultiplexer is provided by the optical device according to the first or second aspect of the present invention. The WDM signal light is supplied to the fiber collimator, and the fiber assemblies are optically connected to the optical receivers, respectively.
In accordance with a fifth aspect of the present invention, there is provided a system comprising first and second terminal units adapted to wavelength division multiplexing, and an optical fiber transmission line connecting the first and second terminal units. Each or at least one of the first and second terminal units comprises the optical device mentioned above. Preferably, this optical device is provided by the optical device according to the first or second aspect of the present invention.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.